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Subject: Re: [boost] [lockfree::fifo] Review
From: Helge Bahmann (hcb_at_[hidden])
Date: 2009-12-11 10:35:08

On Fri, 11 Dec 2009, Andy Venikov wrote:

> Helge Bahmann wrote:
>> This is however NOT possible because you are not allowed to make any memory
>> access between ll and sc -- some CPUs (definitely older mips, but not ppc,
>> unsure about many of the other architectures) clear the reservation on
>> _any_ memory reference and will cause this sequence to always fail. Alpha
>> has the fun requirement of a memory barrier before you are allowed to
>> dereference "tmp", which just provides another way to kill your
>> reservation.
> PPC as well. 7447, for example, will invalidate the "reservation bit" after
> writing to ANY memory location.

Cool, this makes the whole thing model-specific behavior and thus even
more fun :) On the ppc 7455 available to me it not only works, but the
"stwcx." in fact behaves more like a CAS (it remembers the value loaded by
lwarx, and succeeds if it matches -- thus ABA detection won't work for
entirely different reasons).

Other fun restrictions that I remember from some architecture manual (I
don't know which, think it was also some mips flavor): No taken branch
allowed between ll/sc.

The message should however be clear: ll/sc is *brittle* and not usable for
ABA prevention.


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